The subject matter described herein relates generally to medical imaging systems, and more particularly, to a time-to-digital converter (TDC) utilized in a medical imaging system.
Radioactive tracers called “radiopharmaceuticals” are often used to perform medical imaging. As the radioactive tracer decays, the radioactive tracer emits positrons. The positrons travel a very short distance before they encounter an electron, and when this occurs, the positrons are annihilated and converted into two annihilation photons, or gamma rays. This annihilation is characterized by two features which are pertinent to positron emission tomography (PET) imaging systems. First, each gamma ray has an energy of 511 keV and second, two gamma rays are directed in nearly opposite directions. An image is generated by determining the number of such annihilations at each location within the field of view.
To generate the image, at least one known PET imaging system includes a detector ring assembly which converts the energy of each 511 keV photon into a flash of light (scintillation photons) that is sensed by a light sensor. Coincidence detection circuits connect to the detectors and record only those photons which are detected approximately simultaneously by detectors located on opposite sides of the patient, referred to as coincidence events.
To accurately determine coincidence events and thereby obtain useful information for generating the image, PET imaging systems utilize timing circuits to accurately identify and log the times at which photons are received at the detectors. The timing circuits typically include digital counters and digital counter latches that receive both the count signals from the digital counters and impulse signals from the detector whenever photons are detected. Based upon the count signals, the counter latches effectively time-stamp the impulse signals with times indicative of when the impulse signals are received, and output this information for use by the PET imaging system in determining coincidence events. However, known timing circuits, including the digital counters, are relatively complex and may result in an increase in cost to fabricate.